Several processes and techniques for manufacturing a crystalline wafer by transferring layers are generally known. These include, for example, the layer transfer technique reported in Frontiers of Silicon-on-Insulator, J. Appl. Phys. 93, 4955 (2003) by G. K. Celler et al. and based on the “SMART-CUT®” technology of Soitec S.A., which is known to those skilled in the art and descriptions of which can be found in a number of works dealing with wafer reduction techniques, such as U.S. Pat. No. 5,374,564. In the SMART-CUT® process, atomic species, such as ions, are implanted in a donor substrate to create a region of weakness therein before bonding of a handle substrate to the donor substrate. After bonding, the donor substrate splits or is cut at the region of weakness. What is obtained therefore is, on the one hand, a donor substrate, stripped of a layer of its structure, and, on the other hand, a wafer comprising, bonded together, a removed thin layer of the donor substrate and the handle substrate.
It is also known that a region of weakness can alternatively be formed in a donor substrate by forming a porous layer therein using the method known as the ELTRAN® process by Canon, described in U.S. Pat. No. 6,100,166. Additionally, various bonding techniques are generally known and include the method described in the reference entitled “Semiconductor Wafer Bonding: Science and Technology” (Interscience Technology) by Q. Y. Tong, U. Gösele and Wiley.
Layer transfer processes, for example SMART-CUT® processes, advantageously produce crystalline wafers or other structures that preferably include a thin layer of semiconductor material, such as SeOI (Semiconductor-On-Insulator), SOI (Silicon-On-Insulator), and SGOI (Silicon-Germanium-On-Insulator) structures and the like. The resulting structures from such processes are generally used for applications in the field of microelectronics, optics and/or optronics.
The term “implanting” atoms is conventionally understood to mean any bombardment of atomic species, including molecular and ionic species, suitable for introducing the species into the material of a wafer, with the implanted species having a concentration maximum at a predetermined depth within the wafer relative to the bombarded surface, so as to define a region of weakness. The region of weakness is a function of the nature of the implanted species and the implantation energy associated therewith. As will be stated hereafter, however, and within the context of the invention, implantation of atomic species is not limited to conventional bombardment implantation methods, but also extends to any method suitable for introducing atomic species into the donor substrate. In particular, implanting atomic species also includes exposing the wafer to a plasma containing the implantation species to form the region of weakness.
When implanting atomic species in a wafer by bombardment, co-implanting two different atomic species therein advantageously reduces the necessary dose of implantation by a factor of approximately 2 to 3 relative to the implantation of a single type of atomic species. For example, it is established in the article by Aditya Agarwal et al., “Efficient Production of Silicon-On-Insulator Films by Co-Implantation of He+with H+”, Applied Physics Letters, vol. 72 (1998), pp. 1086-1088, that the co-implantation of hydrogen and helium enables thin layer detachment at a much lower total implantation dose than that required when either hydrogen or helium alone is implanted. This reduction of required dose translates to a reduction in the required implantation time, and also to costs associated with production of wafer structures comprising a thin layer on a handle substrate, in particular by means of a transfer process, such as Soitec's SMART-CUT® process.
Co-implantation of atomic species, however, also presents a disadvantage that blisters tend to form at the bonded interface between the free surface of the implanted donor substrate under which implantation has been carried out and the surface of the handle substrate. Formation of blisters are especially prevalent during certain additional operations, such as thermal treatments, that are commonly performed during a SMART-CUT® layer transfer process. It is known that blisters tend to form after a co-implantation of atomic species, for example helium and hydrogen species, because helium species may diffuse in the matrix of the donor substrate more easily than hydrogen species, and the risk of blister formation increases if helium is implanted close to the bonded interface.
The appearance of blisters at the bonded interface may effectively lead to the degradation of the bonded interface. Hence, when a SMART-CUT® process is carried out, blisters that form at the bonded interface may disturb the structural properties of the thin layer which has been detached. Blisters may even cause a detachment at the level of the blister sites, that is at the level of the bonded interface and not at the level of the region of weakness, thus creating “non-transferred” zones and introducing roughness and structural defects to the transferred thin layer. Structures presenting non-transferred zones are usually rejected from the production line, and hence decrease the production yield.
Additionally, blisters and voids have also been observed to some extent when an implantation is carried out by implanting a single atomic species within a donor substrate. This problem, for example, is usually encountered when producing an SOI structure that includes a thin layer of buried oxide having a thickness below about 500 angstroms, as described in U.S. Patent Application Publication No. 2004/0248380. A similar problem is also observed when direct silicon-silicon bonding occurs during the SMART-CUT® process.
Previous methods have been used to avoid blister formation. A first method includes implanting helium species deeper within the donor substrate than hydrogen species, with respect to the free surface of the donor substrate under which implantation is performed. Generally, it has been found that the deeper the helium species are implanted with respect to the hydrogen species, the less blister formation is observed at the bonded interface.
A second method of reducing blister formation includes increasing the dose of hydrogen species that are implanted, typically by a dose from about 2 to 5×1015/cm2. Generally, it has been found that the higher the hydrogen dose that is implanted, the less blister formation is observed at the bonded interface. In both methods, the region where hydrogen species are implanted is regarded as acting as a gettering region or barrier making it possible to block the diffusion of helium species towards the bonded interface.
In addition to concerns regarding the formation of blisters at the bonded interface, the resulting surface roughness of the thin layer of the wafer that is newly formed after SMART-CUT® processing is also a consideration. As mentioned above, the donor substrate is typically detached at the region of weakness created by the implantation step so as to transfer a part of the donor substrate onto the handle substrate, and to form the thin layer on the handle substrate. The specifications of the surface state of structures obtained by a layer transfer process such as SMART-CUT® are generally very strict. The surface roughness and the thickness uniformity of the thin layer are parameters which condition the quality of the components that are created on the structure.
In general, it has been found that after co-implantation of helium and hydrogen species in a donor substrate to create a region of weakness, the resulting surface roughness and thickness uniformity of the thin layer are most favorable (i.e. exhibit a low surface roughness and uniform thickness) when the distance between the implanted helium and hydrogen species is minimized, and when the dose at which hydrogen species are implanted is minimized. Hence, certain implantation conditions that result in the exhibition of more favorable surface roughness and uniform thickness may lead to the undesired formation of blisters, and reciprocally, conditions that avoid blister formation may result in poor surface roughness and thickness.
Due to the fact that surface roughness, thickness uniformity, and blister formation cannot be controlled separately, a compromise is typically made between employing the most favorable conditions (i.e. implantation energy for controlling implantation depth, and dose of implanted species) for avoiding blister formation and the most favorable conditions for both limiting the resulting surface roughness and obtaining a suitable thickness uniformity. By carrying out such a compromise, however, it is extremely difficult to produce a structure having an optimal surface roughness and uniformity on the one hand, and at the same time optimally avoiding the formation of blisters at the bonded interface.
Thus, there is a need for a method for producing a high quality crystalline wafer or structure that includes a thin layer of material on a substrate without compromising implantation conditions for avoiding blister formation and implantation conditions for both limiting the resulting surface roughness and obtaining a suitable thickness uniformity.